The recent increase in concern with a global environmental problem and safety of the human body has boosted the demand for less hazard and more safety, as well as flame retardancy, of electric/electronic products. That is, reduction of harmful gas and smoke generation has been demanded.
Use of a bromine-containing flame retardant, which provides good flame retardancy, has been being limited on account of generation of toxic hydrogen halide gas (hydrogen bromide) on combustion. Therefore, studies have been directed to compositions containing an ordinary epoxy resin and a halogen-free flame retardant, such as a nitrogen compound, a phosphorus compound, or an inorganic compound. However, these additives for imparting flame retardancy are disadvantageous in that their flame retardant effect is insufficient or they adversely affect curing of the epoxy resin or are accompanied by reduction of physical properties, such as a glass transition temperature, of a cured product.
For example, triphenyl phosphate is widely used as a phosphorous-containing flame retardant for various resins. Patent Document 1 described below proposes adding a phosphorus-containing flame retardant composed of a high-molecular-weight divalent phenol and a phenol to an epoxy resin. Nevertheless, the proposed flame retardant should be used in a large quantity in order to impart sufficient flame retardancy to an epoxy resin. Adding a sufficient amount of the flame retardant to realize satisfactory flame retardation causes reduction in glass transition temperature, and reduction in amount of the flame retardant to be added in an attempt to increase the glass transition temperature results in insufficient flame retardancy. Patent Document 2 below proposes using a reactive phosphoric ester compound. The proposal has turned out to be impractical because incorporating the phosphoric ester compound into an epoxy resin makes the resin hygroscopic or makes part of the resin take on a three-dimensional structure, which increases the viscosity and thereby greatly reduces the workability of the resin composition.